1. Technical Field
The present invention relates to an apparatus for sorting out conforming granular objects from non-conforming granular objects by color.
2. Background Art
There is well known a color sorting machine which projects light onto objects to be sorted (granular objects) serving as a material, senses the difference between the amount of light from each object to be sorted and the amount of light of a background that serves as a reference with, e.g., an optical sensor, and screens out granular objects without a predetermined color (a predetermined amount of light) with an ejector, such as an air injection nozzle, on the basis of detection signals from the optical sensor. Among the screened-out granular objects without the predetermined color are some normal granular objects (conforming grains). For this reason, a secondary sorting unit which picks out conforming grains from among granular objects discharged through a non-conforming grain outlet is generally provided to improve the rate of screening. The rate of screening refers to the ratio of the total weight of non-conforming grains mixed in granular objects separated and fed to a non-conforming grain port to the total weight of non-conforming grains mixed in granular objects as a material.
As shown in simplified form in FIG. 6 of the present application, Japanese Patent No. 3044701 discloses a granular object color sorting machine in which a conveying surface 101 of a belt conveyor is divided by a partition wall 102 to form a conveying surface 103 for secondary sorting, a vibrating supply trough for secondary sorting (a feeder not shown), a detection unit 104, and an ejector 105 are provided, comparators having positive reference voltages (threshold values) are provided in a control unit for primary sorting and secondary sorting, respectively, and switches are respectively provided to output or stop signals from the comparators.
With the above-described configuration, primary sorting and secondary sorting are performed in parallel by supplying granular objects removed by primary sorting and separately fed to a non-conforming grain outlet to the conveying surface 103 for secondary sorting formed by the partition wall 102 on the belt conveyor. If there are a small number of normal grains (conforming grains) among the granular objects removed by primary sorting and separately fed to the non-conforming grain outlet, in secondary sorting, the current reference voltage is switched and set to a reference voltage opposite in polarity to one in primary sorting by a switch, and the conforming grains smaller in number than non-conforming grains are removed by the ejector (reverse screening).
The disclosed color sorting machine can set a positive reference voltage or a negative reference voltage according to the rate of mixed non-conforming grains in primary sorting and secondary sorting, use different screening sensitivities for primary sorting and secondary sorting, or set “reverse screening” described above in secondary sorting.
However, the use of different screening sensitivities in a primary sorting unit and a secondary sorting unit and setting of “reverse screening” in the secondary sorting unit cannot be said to be a fundamental solution to a reduction in yield. More specifically, in the primary sorting unit of the color sorting machine, the rate of non-conforming grains mixed in granular objects separated and fed to a conforming grain side is low. In other words, the primary sorting unit removes non-conforming grains without fail with an emphasis on the quality of granular objects separated and fed to the conforming grain side and thus often screens out and removes conforming grains together with non-conforming grains. Similarly, the secondary sorting unit often screens out and removes conforming grains together with non-conforming grains. The rate of conforming grains mixed in non-conforming grains increases, which leads to a declining trend in yield. The cause of the trend will be described with reference to FIGS. 7A and 7B.
FIGS. 7A and 7B show the ejector 105 in a multi-channel color sorting machine. A gap between two adjacent air injection nozzle ports 105a and 105b is formed to be as narrow as about 1.0 mm. As shown in FIG. 7A, the conventional color sorting machine is set so as to simultaneously operate the two air injection nozzle ports of the air injection nozzle port 105a and the air injection nozzle port 105b in proximity to a conforming grain to inject air and remove a non-conforming grain in order to prevent a failure to remove the non-conforming grain when the non-conforming grain in proximity to the conforming grain passes near the air injection nozzle port 105a. This leads to a declining trend in yield. If only the one air injection nozzle port 105b is operated when a non-conforming grain passes astride the two air injection nozzle ports 105a and 105b, as in FIG. 7B, air cannot be injected to a center position (barycentric position) of the non-conforming grain, and a failure to remove (eliminate) the non-conforming grain occurs. The color sorting machine is thus set so as to prevent a failure in removal by simultaneously operating the two air injection nozzle ports, i.e., the air injection nozzle port 105a together with the air injection nozzle port 105b to inject air to the center position (barycentric position) of the non-conforming grain. However, if a conforming grain is passing on the air injection nozzle port 105a side at the time of the simultaneous operation, the conforming grain is involved in the removal of the non-conforming grain and is also removed. This leads to a declining trend in yield.